The present invention relates to a method for the preparation of a sintered body of silicon carbide or, more particularly, to a method for the preparation of a sintered body of silicon carbide having high electric insulation and thermal conductivity and useful as a material of electronic components such as substrate plates of integrated circuits.
As is well known, sintered bodies of silicon carbide are widely used in a variety of applications by virtue of their excellent properties such as heat resistance, abrasion, resistance, mechanical strength and corrosion resistance. A recently highlighted application of sintered bodies of silicon carbide having high electric insulation and thermal conductivity is as a material of various electronic components such as substrate plates of integrated circuits.
It is however, technologically a difficult matter to remarkably increase the electric resistivity and thermal conductivity of sintered bodies of silicon carbide. For example, a method is proposed to admix the silicon carbide powder with beryllium oxide as a sintering aid. A difficult problem in this method is that beryllium compounds in general have strong toxicity to cause adverse effects on the workers' health unless the material is handled with utmost care. Moreover, the effect of beryllium oxide as a sintering aid is relatively small so that sintered bodies cannot be imparted with a satisfactorily high density when sintering is performed under normal pressure necessitating use of a very expensive apparatus for hot-pressing. In addition, scantiness of the resources of beryllium in nature greatly limits the supply of beryllium oxide which is necessarily very expensive.
On the other hand, most of the currently used heat-resistant substrate plates of integrated circuits are made of a sintered body of aluminum oxide in view of the inexpensiveness while aluminum oxide has a thermal conductivity of 20 W/m.K and a coefficient of thermal expansion of 8.times.10.sup.-6 /.degree.C., which value is considerably larger than the value of 3.5.times.10.sup.-6 /.degree.C. of the single crystal of semiconductor silicon. Accordingly, it is eagerly desired to develop a highly heat-radiative material to replace aluminum oxide. Besides silicon carbide, one of the promising candidate materials is aluminum nitride but aluminum nitride is not free from serious disadvantages of greatly limited fields of applications due to the inherently poor thermal conductivity and low resistance against chemicals.
U.S. Pat. No. 4,209,474 teaches a method for the preparation of a sintered body of silicon carbide having N-type semiconductivity, according to which a powder of silicon carbide is admixed with a boron additive and the powdery mixture is shaped and sintered at a temperature in the range from 1280.degree. C. to 2300.degree. C. in an atmosphere in which the partial pressure of nitrogen is at least 10.sup.-1 atmosphere. Although this method is effective in obtaining the desired semiconductivity, no satisfactory sintered body of silicon carbide can be obtained when application thereof is desired as a material of electrically insulating substrate of integrated circuits.